1. Field of the Invention
The present invention relates to structural assemblies and, more particularly, relates to an apparatus and associated method for forming and heating treating structural assemblies.
2. Description of Related Art
Superplastic forming (“SPF”) is a process used to form structural assemblies having complex three-dimensional shapes, such as the two- and three-sheet assemblies 11, 12 illustrated in FIGS. 1A and 1B, respectively. These assemblies are formed from plastics, metals and metal alloys, including aluminum, titanium, and alloys thereof, that exhibit superplastic behavior at certain temperatures, i.e., large elongations (up to 2,000 percent).
During the SPF process, an “SPF pack” (also referred to herein as a “preform”) is formed of a single sheet or two or more stacked sheets of material. The SPF pack is placed into a shaping die and heated to a sufficiently high temperature within the superplasticity range of the material to soften the sheet or sheets of material. Pressurized heated gas is then injected against and/or into the SPF pack, causing the pack to inflate and fill the die, thereby forming a structural assembly Once inflated, the portions of any sheets of material that are in contact with an adjacent sheet of material will join through brazing or diffusion bonding. The structural assembly is then cooled and removed from the die and final machining steps are performed, such as edge trimming, to form the finished structural assembly. Advantageously, the SPF process can be used to form structural assemblies that can satisfy narrow shape and tolerance requirements without substantial additional machining. Such combined cycles for forming are described in U.S. Pat. No. 5,410,132 entitled “Superplastic Forming Using Induction Heating,” which issued on Apr. 25, 1995; U.S. Pat. No. 5,420,400 entitled “Combined Inductive Heating Cycle for Sequential Forming the Brazing,” which issued on May 30, 1995; U.S. Pat. No. 5,700,995 entitled “Superplastically Formed Part,” which issued on Dec. 23, 1997; U.S. Pat. No. 5,705,794 entitled “Combined Heating Cycles to Improve Efficiency in Inductive Heating Operations,” which issued on Jan. 6, 1998; U.S. Pat. No. 5,914,064 entitled “Combined Cycle for Forming and Annealing” which issued on Jun. 22, 1999; and U.S. Pat. No. 6,337,471 entitled “Combined Superplastic Forming and Adhesive Bonding” which issued on Jan. 8, 2002, each of which is assigned to the assignee of the present invention and is incorporated herein by reference.
For some materials, the high temperatures of the SPF process reduce the material properties, such as strength, toughness, and corrosion resistance, of the material. Thus, a subsequent heat treating process may be required after the structural assembly has been superplastically formed in order to restore these material properties. For example, structural assemblies formed of aluminum alloys often require heat treatment, such as quenching, after superplastic forming to restore the strength and toughness of the material. Undesirably, temperature gradients in the structural assembly during the heat treating process can result in shape and dimensional distortion of the assembly, which can require further machining and/or forming operations to bring the assembly back into compliance with the required shape and dimensional tolerances. These additional machining and forming operations can be time and labor intensive, which can adversely affect the overall cost of the finished assembly.
Thus, there exists a need for an apparatus and associated method for producing complex structural assemblies that exhibit high material properties, including high strength, toughness, and corrosion resistance, and that can consistently satisfy narrow shape and dimensional tolerances, as are typically required in the aerospace and automotive industries. The apparatus must be cost effective, and should be easily adaptable to variations in the configurations of the structural assembly.